The present invention relates generally to thermoplastic molding compositions which comprise a blend of an ABS copolymer and a copolymer of an unsaturated dicarboxylic acid anhydride, and a minor portion of an alkylene oxide copolymer. The present invention also relates to such thermoplastic compositions wherein the blend additionally includes a polycarbonate.
Blends of ABS copolymers and unsaturated dicarboxylic acid anhydride copolymers, alone or in combination with other resins such as polycarbonates, are well known and commercially available as thermoplastic molding compositions. These compositions are desirable for many uses due to their improved dimensional stability at elevated temperatures in comparison to many conventional ABS or polystyrene compositions, and may be molded into a variety of useful articles, such as automobile grills and trim, housings for telephones and computers, and other household and business appliances.
A part molded from such a composition should have a smooth, even surface appearance, as covering the part surface to hide surface defects, such as by painting, adds to the cost of the product. Even when a part will necessarily be plated or coated prior to use, such as "bright work" on automobiles, surface defects in the part may be translated through the plating or coating to the part's final finish, making the part unacceptable. Although parts may be rejected merely for aesthetic reasons, surface defects, such as splay or streaking, are also a concern because they may signify the occurence of degradation in the plastic composition, resulting in weakening of the part's physical properties. For these reasons parts which display significant surface defects are usually discarded, thereby increasing the overall cost of the product.
Although virtually all thermoplastic compositions will exhibit surface defects if processed under conditions which are sufficiently severe, this problem is particularly acute with resins containing unsaturated discarboxylic acid anhydrides, especially when in combination with nitrile containing resins.
ABS/anhydride copolymer blends also often exhibit a pronounced tendency, when melted, to "stick" or adhere to metal surfaces, such as those commonly found in mixing and molding equipment. Stickiness and surface defects in such resins frequently are the result of the occurence of crosslinking, polymer degradation and volatile evolution in the resin composition. Such phenomena are particularly pronounced at elevated temperatures, such as those at which the composition is mixed and molded, and are a significant deficiency in current commercial anhydride/ABS blends.
Degradation or splay at elevated temperatures is a significant disadvantage because many molders run their molding equipment at as high a temperature as is consistent with molding an acceptable product in order to maximize the speed of the molding operation. A product which develops significant splay at high temperatures is therefore much less desirable than one which can be molded at a higher temperature without developing significant surface defects.
Adhesion of the blend to metal surfaces may be disadvantageous for several reasons. As is readily apparent, adhesion or "sticking" usually will increase the time required to achieve adequate mixing of the blend. As at least some of the factors causing surface defects are dependent upon time and temperature, prolonged mixing may contribute to surface defects, as well as decreasing through-put of the mixing operation. Material which remains stuck inside equipment after mixing or molding may degrade further, thereby necessitating cleaning of the equipment prior to processing of more material. Sticking is a particular problem with Banbury mixers and roll mills, and may also be a problem with twin screw extruders. Particularly in view of the fact that Banbury mixers and roll mills are used in a substantial portion of current resin and part production, an ABS/anhydride polymer blend with a reduced tendency to adhere to metal surfaces and a reduced tendency to develop surface defects offers many significant practical advantages.